1. Field of the Invention
The present invention relates to head gimbal assemblies and servo controller of a hard disk drive.
2. Background Information
Hard disk drives contain a plurality of magnetic heads that are coupled to rotating disks. The heads write and read information by magnetizing and sensing the magnetic fields of the disk surfaces. There have been developed magnetic heads that have a write element for magnetizing the disks and a separate read element for sensing the magnetic field of the disks. The read element is typically constructed from a magneto-resistive material. The magneto-resistive material has a resistance that varies with the magnetic fields of the disk. Heads with magneto-resistive read elements are commonly referred to as magneto-resistive (MR) heads.
Each head is embedded in a slider, which is attached to a flexure arm to create a subassembly commonly referred to as a head gimbal assembly (HGA). The HGA's are attached to an actuator arm. The actuator arm has a voice coil motor that can move the heads across the surfaces of the disks.
Information is stored in radial tracks that extend across the surfaces of each disk. Each track is typically divided up into a number of segments or sectors. The voice coil motor and actuator arm can move the heads to different tracks of the disks and to different sectors of each track.
A suspension interconnect extends along the length of the flexure arm and connects the head to a preamplifier. The suspension interconnect typically comprises a pair of conductive write traces and a pair of conductive read traces. One pair of traces, such as the read traces, extend down one side of the flexure arm to the head and the remaining pair of traces extends down the other side of the flexure arm to the head.
The Tracks Per Inch (TPI) in hard disk drives is rapidly increasing, leading to smaller and smaller track positional tolerances. The track position tolerance, or the offset of the read-write head from a track, is monitored by a signal known as the head Positional Error Signal (PES). Reading a track successfully usually requires minimizing read-write head PES occurrences. The allowable level of PES is becoming smaller and smaller. A substantial portion of the PES is caused by disk vibration.
Track Mis-Registration (TMR) occurs when a read-write head tends to lose the track registration. This occurs when the disk surface bends up or down. TMR is often a statistical measure of the positional error between a read-write head and the center of an accessed track. Bending is defined in terms of bending modes. For a positive integer k, a bending mode of (k,0) produces k nodal lines running through the disk surface center, creating k peaks and k troughs arranged on the disk surface. Bending mode (0,0) produces no nodal lines, either the entire disk is bent up or bent down.
Two basic prior art approaches are known to lower the Track Mis-Registration (TMR) due to disk vibration. One approach uses head gimbal assemblies providing a radial motion capability. The other approach alters the servo-controller to reduce TMR.
In the first approach, a head gimbal assembly, including a biased load beam, creates a roll center (also known as a dimple center), which provides a radial motion capability as the load beam moves vertically due to disk vibration. This allows sliders to move in a radial direction as well as in a vertical direction with respect to the disks, reducing off-track motion due to disk vibration.
The first approach has some problems. An air bearing forms between the slider face and the disk surface. The slider face is tilted near the disk surface when it is flat. The air bearing becomes non-uniform when the disk surface is flat, adding new mechanical instabilities into the system.
One alternative prior art head gimbal assembly provides a slider mounted so that it pivots in the radially oriented plane about the effective roll axis, which is located within the disk. This scheme does not cause a non-uniform air bearing when the disk surface is flat. However, the way the effective roll axis is placed inside the disk requires a more complex mechanical coupling between the slider support assembly and the slider. This complex mechanical coupling may have a greater probability of mechanical failure, tending to increase manufacturing expenses and to reduce hard disk drive life expectancy.
The second prior art approach to lowering TMR due to disk vibration alters the servo-controller. These servo controllers favor optimization of PES in the disk vibration range without regard for strengthening rejection of low frequency disturbances. The disk vibration range will be considered to include frequencies between about 1K Hz and about 4K Hz. Low frequency disturbances will be considered to include at least the frequencies between about 0 Hz and about 800 Hz.
Accordingly, there exists a need for head gimbal assembly mechanisms providing a stable air bearing, able to follow a track when a disk surface bends, which are easy and reliable to manufacture. There exists a need for servo controllers optimizing PES in the disk vibration range and taking into account potential advantages from strengthened rejection of low frequency disturbances.